Constructing a Raised Bed Garden

Lay out the Perimeter

If the bed has straight lines, use stakes and string to outline the perimeter. Garden hose or rope works well for outlining curved beds. Most vegetable beds are square or rectangular so that vegetables can be planted in rows. Many ornamental beds are curved. To make maintenance easier, particularly mowing, design the bed with long, flowing curves rather than many tight ones.

Remove Existing Vegetation

Remove any woody plants with loppers, hand saws or chainsaws, then dig out the roots. Apply a systemic herbicide to kill perennial weeds and prevent them from returning. Or, kill vegetation without herbicide by covering the bed area with clear plastic (anchor edges with rocks or soil) for 1 to 2 months. If both day and night temperatures are warm, the intense heat generated under the plastic will kill plants, though not as quickly as herbicide. Once the site is bare of vegetation, till the soil thoroughly.

Install Edging

Metal. Metal edging is usually packaged as 4- to 6-inch wide metal strips in varying lengths. They are connected and held together by stakes inserted through overlapping notches. Place the strips on edge along the perimeter of the bed and overlap the ends, lining up the notched strips. Hammer the stakes into the soil through the overlapping notched strips. Using a rubber mallet or a piece of wood between a hammer and the top of the edging, lightly hammer the edging into the soil between the stakes. It is best to partially sink the stakes until all are in, and then sink them to the desired depth. If the soil is hard and dry, soak it before installing the edging, or excavate the soil to accommodate the edging.

Brick/Cinder Blocks. To build a brick-edged raised bed, first pour a concrete footer at least 6 to 12 inches high and 16 to 18 inches wide. This will be the base of the wall. Dig the trench for the footer carefully so that you wonıt need to use forms. Once the concrete is poured, work a 3/8 -inch reinforcing rod into the center for stability (especially important in clay soils). Smooth the top of the footer with a trowel. After the footer has cured for 3 or 4 days, wet it and apply about 3/4 to 1 inch of mortar about 2 feet down the slab. Press the first brick into the mortar so that about 1/2 inch of mortar is left between the brick and slab. Apply mortar to the side of the next brick and place it 3/8 inch from the first one. Rap the brick gently with the trowel handle to set it and remove the excess mortar squeezed from between the bricks. Continue until the edging is complete. Cinder block edging may not require mortar because the blocks are larger.

Stone. To raise the stones, roll them up a plank on pipes or use a hydraulic lift. To install a dry stone wall, first level the terrain of the perimeter. Lay the stones in each row so that they overlap the stones underneath. Make the wall wider at the base than at the top, and cant the stones inward for stability. If the wall is to be more than one stone thick, periodically insert tie stones‹long stones laid across the width of the wall. This makes the wall stronger. If the wall is to be more than 2 feet high it should be mortared in place and built on a concrete footer. Test fit two or three stones at a time before applying the mortar. To ensure good contact between the mortar and the stone, lay the first layer of stone while the footer is still wet and rap the stone sharply with the trowel handle to set it. If the stones are heavy, insert wooden pegs between the stones to keep the mortar from being squeezed out before it dries. Remove the wooden pegs after the mortar has partially set and fill the holes with mortar. It may be advisable to have expert help because an improperly constructed stone wall can be hazardous. Also check local ordinances to determine whether an architectıs seal is required for the plans.

Landscape Timbers. Level the perimeter of the bed so that the first layer of timbers is level or set at the desired slope. Drive rebar or galvanized spikes through the ends of the timbers at 20-degree angles to the center of the timber, and then drive them into the soil approximately 12 to 18 inches. Lay the successive layers of timbers in an overlapping fashion and nail them to the previous layer with galvanized spikes. Check the level or desired slope frequently during construction.

Install Irrigation System

If you will have drainage trenches or an automatic sprinkler system, install them before soil is added to the bed.

Add Soil

To help keep out aggressive lawn grasses, especially bermudagrass, install a weed barrier between the edging and the soil. Then you are ready to add soil or growing media.

Soil should hold water well so that plant roots do not dry out, but it should also have good drainage. Soil with too much sand does not hold water well; soil with too much clay does not drain well. Generally, a sandy clay loam soil is best for most plants. It should be mixed with organic matter such as peat moss, composted manure, sawdust or ground bark.

Soil is sold and delivered in cubic yards. It can be ordered as a topsoil/compost blend; common mixtures are three-fourths topsoil and one-fourth compost, two-thirds topsoil and one-third compost, and half topsoil and half compost. The higher the organic material content the sooner you will need to add more soil/compost to the bed, because the organic matter breaks down over time. Make sure organic material has been composted before it is added to the soil. Otherwise, it will deprive plants of nitrogen as it decomposes. The best media for vegetables consists of one-third topsoil, one-third peat moss and one-third sand or coarse perlite. Standard potting soil or commercial container mixes are also good for growing vegetables, but are usually too expensive for filling large beds. When filling the bed, grade the soil so that it slopes slightly away from the center of the bed to the edge, and away from adjacent structures.

It can be difficult to incorporate existing trees or shrubs into a raised bed. The easiest method is to encircle the plant with metal edging to keep soil and excess mulch away from the crown of the plant. Leave as wide a space as possible between the edging and the plant. Tree wells can be used in taller beds. It is important to remember, though, that adding large quantities of soil over the roots of established plants is not advisable and may kill the plants.